Telephony.



M. L. JOHNSON.

TELEPHONY.

APPLICATION HLED MAY 22,1911.

Patented Mar. 18, 1913.

2 SHEETS-SHEET l.

M. L. JOHNSON.

TELEPHONY. APPLICATION FILED MAY 22, 1911.

, UNITED STATES PATENT OFFICE.

MORTON LJJOHNSON, OF CHICAGO, ILLINOIS, ASSIGNOR TO COBWIN TELEPHONE MAN- UFACTURING COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

TELEPHONY.

To all whom it may concern:

Be it known that I, Monron L. Jomsson, a citizen of the United States, residing at Chicago, in the couny of Cook and State of Illinois, have invented a certain new and useful Im rovement in Telephony, of which the following is a full, clear, concise, and exact description, reference being bad to the accompanying drawings, forming a part of lit this specification.

My invention relates to telephone exchange svstems employing party telephone lines an includes features which may be of service in connection with manual, automatic and semi-automatic exchange systems. I em loy electro-magnets at the party telephone line stations for preventing and permittin the operation of selecting mechanism at tie exe ange where the exchange employs mechanism for automatically extending lines, and for governing the switches that control the circuit association of the telephone receivers with the line, there being a source of current normally connected wit the line to the current from which the electro-magnets are normally responsive to enable the party at a station first seeking the use of the line effectively to manipulate his apparatus for that purpose. There is present mechanism for changing the circuit association of the source of current with the line when the line has been taken for use so that arties at other stations upon the line who liiter seek the use of the line cannot include their telephone receivers in circuit association with the line nor cause nor interfere with the operation of the selecting mechanism at the exchange.

The electro-magnets are desirably included in high resistance bridge conductors while there are low resistance bridge conductors at the difierent stations to enable a party line subscriber who takes the line for use to establish a shunt to de rive the magnets at other stations of opera ing current so that parties at such other stations later seeking to take the line cannot interfere with its use.

I will explain my invention more fully by reference to the accom anyin drawings showing an embodiment t ereof 1n the form of an automatic telephone exchange system, to which the invention is not to be limited, and in which drawings- Figure 1 shows a party telephone line with Specification of Letters Patent.

Application filed Kay 22, 1911,

Patented Mar. 18, 1913.

Serial No. 628,777.

four stations connected therewith, and Fig. 2 shows the exchange end of said party telephone line and automatically operated seecting mechanism for effecting extension of the party telephone line.

Tie telephone line extends from the exchange in two limbs 1, 2 and, in the embodiment of the invention illustrated, is normally connected at its two sides with the poles of a battery 3, the side 1 being grounded and connected through the ground to the positive pole of the battery 3, and the negative pole of the battery 3 being connected with the side 2 of the line. The current furnished by the batter 3 is preferably uni-directional, articular y where the invention is embodied in an automatic or semi-automatic telephone exchange system, the source of uni-directional current being normally connected with the line to permit current to have initial flow over the line in a single direction. Mechanism is provided at the exchan e for reversing the current applied to the ins after a party at one of the telephone stations upon the line takes the line for use, the source of current 3 bein itself then reversed so that the side 1 is c iarged from the negative pole of the battery and the side 2 is charged from the positive pole of the battery. This is the case where the source of current 3 furnishes uni-directional current, but I do not wish to be limited to the employment of the same source of current for connection with the line normally and after the line has been taken for use. The a paratus by which the battery 3 is reverse and which is illustrated in Fi 2, is familiar to those that are skilled in t 1c art of automatic telephony and will be later more fully referred to.

I have shown four party line stations A B C D though I do not limit myself to the number of stations that are connected with the party line. As the invention is illustrated, t e equipments at the four stations are identical exce t, if desired, the signal receivers at the di erent stations may be of harmonic ty e to respond to wave form current of di erent periodicities to be selectively im ressed u on the line at the exchan e. he si na receivers may be alike, if desired, in which event code signals should be employed.

Similar parts at the different stations will be given similar characters of reference, one

station being specifically described. In the system illustrated, being an automatic telephone exchange system, I have illustrated a well known type of switching mechanism at each telephone station for operating the selectin mechanism at the exchange. In the embodiment of the invention illustrated, the switching mechanism is controlled b a well known dial mechanism that inc udes a spring winding finger ring or dial 4. The lever 5, enga ed by the finger at the limit of the dial stro e, releases a spring actuated device that intermittently operates the circuit opening switch 6 durin the return of the spring actuated mechanism to normal. It is not necessary to describe in further detail the switching mechanism that is employed to operate the selecting mechanism at the exchange and furthermore, the invention is not to be limited to the particular type of switching mechanism illustrated, nor is the invention in all of its embodiments to be limited to a telephone s stem employing selecting mechanism for e ectin extension of the tele hone line. It shou d be said, however, t at when the invention is embodied in a system employin selecting mechanism for automatically e ecting the extension of telephone lines that it is usual, where dial mechanism is used, to employ a master switch including contacts 7 and 8, in conjunction with the switching device 6 and in serial relation therewith, this master switch being controlled by the switch hook 9. The telephone receiver 10, when upon its switch-hook, operates to depress the element 8 of the master switch, and there open the circuit throu h switch 6, through the intermediation o the depressing finger 11 and the right hand end portion of the lever 12 then depressed by the finger 11 the right hand end portion of the lever 12 lacing mechanically connected with the switching element 8 by means of a strut 13. The switch hook when depressed causes pawl 11 to hold the dial. There is a low resistance bridge conductor at each telephone station that includes the tele hone transmitter 14 havin one terminal te ephonically connected with tie limb 1 of the tele hone line, and the receiver 10 in serial re ation with the transmitter 14, the dial switch 6, and the contacts 7 and 8.

The normally open connection which has just been traced etween the sides of the telephone line is in parallel relation with another brid e conductor which is desirably of high resistance and is preferably normally open, at the contacts 15, 10. This second bridge conductor includes a magnet 17 which is preferably polarized, and the entire resistance of th s second bridge conductor is preferably located in the magnet which may be wound, for example, to ten thousand ohms resistance. The armature of the polarized magnet 17 is preferably held in a predetermined normal position by means of a spring 18 that normally locks the nose or angular extension 19 of the armature beneath the nose or angular extension 20 of the intermediately pivoted lever 12. A spring 21 normall presses the nose 20 against the nose 19. T e contact element 16 is mechanically connected with the finger 11 so that said contact 16 is permitted to enga e the contact 15 when the associated telep one receiver 10 at a calling station is removed from its hook, whereby the high resistance bridge conductor including the magnet 17 is closed whereupon current from the battery 3 is permitted initial flow over the line sides in a direction that will enable the magnet 17 to respond and draw its armature 19 from engagement with the nose 20 of the lever 12 against the force of the sprin 18 whereupon the spring 21 is permitted to swing the lever 12 in a counter clock wise direction, the finger 11 having previously been raised owing to the removal of the telephone from its switch-hook. The contacts 7 and 8 of the master switch are thereupon engaged, whereafter the low resistance bridge conductor is intermittently broken at 6 in a manner which is well understood and for the purpose of operating the selecting mechanism at the exchange to extend the telephone line in a manner well known. The low resistance shunts at the stations permit of sufiicient flow of current through the selecting mechanism at the exchange to enable such selecting mechanism to operate. The high resistance shunts that include the magnets 17 do not permit of the diversion of sutiicient current from the low resistance shunts to interfere with the control that the switching mechanism in such a low resistance shunt at a station which has appropriated the line has over the selecting mechanism at the exchange. It is of course understood that the electro-magnet 17 is res onsive only to the current that is initia ly impressed upon the line owing to the normal connection of the battery 3 with said line this magnet at other times maintaining the telephone bridge 0 en.

By mechanism whicli is well understood, and which will later be mentioned more fully. the battery 3 has its connections with the sides of the telephone line reversed so that the current impressed upon the line immediately after the line is taken for use is such that the magnets 17 will not respond thereto, whereby those stations upon the line that are late in seeking to take the line for use cannot ap ropriate the line owing to the fact that the ever 12 at any station later to seek appropriation of the line is locked by the armature of the magnet at such station. whereby the contacts 7 and 8 of the master switch cannot be engaged to close the gap in the low resistance bridge at this later station, which closure is essential to enable a station to take a line for use, pawl 11 then locking the dial. While current at any stat1on at any timemay be permitted to flow through the magnet 17 at such station, yet such magnet wil not respond if the batter 3 has been reversed since the magnet wil then be so energized as merely to tend to move its armature in the same direct-ion in which such armature is pulled by the s ring 18 without effecting readjustment oi the parts that enter into the control of the receiver circuit and the control of the switching mechanism that governs the operation of the selecting mechanism whereb the associate telephone receiver is locke out. I therefore speak of the ma et 17 as being unresponsive to current t at is caused to flow over the telephone line in a direction opposite to the normal flow of the current.

The signal receiver 22 responds to wave form current that is preferab y an alternating current and is operated in a manner which is well understood.

I have described the operation of the equi ment at a single telephone station in detail under the assumption that such a station is a callin station, and under such assumption the attery 8 is immediately reversed after the line has been taken for use. When any of the stations upon the line is a called station, the battery 3 should have its normal connection with the tele hone line reserved, at least until the cal ed station ms responded, in order that the magnet 17 at the called station may respond to ermit the closure of the contacts 7 and 8 t crest and thereby enable the inclusion of the telephone e uipment at the called station in iridge o t e telephone line. The low resistance bridge established at the called station by the removal of the telephone receiver from its switch hook serves to prevent the passage of sutlicient current to operate magnets 17 at the remaining stations, irrespective of the direction of such current, so that none of the remaining stations connected with a called line is able to secure connection with such line when a called station has reviously responded. I do not limit myse f to the function of the low resistance s nut in excluding the magnets 17 from operative association with the current impressed upon the line at the exchange.

I will now describe, in a general way, the means by which the battery 3 is reversed after the line has been taken for use by a calling party and whereby said battery is maintained in normal connection wit) 11 called line, at least until the called part res onds. Referring now more particular y to l ig. 2 the battery 8 has its ne ative pole normally connected with the si e 2 of the telephone line through an energizing winding 23, the'switch spring 24 and the normal contact of such spring. The ositive pole of the battery 3 is connected with the side 1 of the telephone line through ground and the switch spring and its normal contact. When the telephone line is taken for use by a callin party at one of the stations, circuit for the attery 3 is closed at the contacts 15, 16 at that tele hone station, whereupon inag net 17 at sucii station is operated with results which have been described. Later, the contacts 7 and 8 at the line appropriating station are engaged to close the low resistance shunt at such appropriating station, whereupon current from the battery 3 is increased sufiiciently to energize the winding 23 (this winding not having been previously operatively energized owing to the presence 0 the hi h resistance magnet 17 in circuit therewith to cause the armature 26 to be attracted. The armature 26, in being attracted, ushes the contact 27 into engagement wit the normal contact of the switch spring 25, thereby to bring the energizing winding 28 into a closed local circuit that may be traced from the grounded normal contact of the switch spring 25 through the windin 28 to the rounded battery 3 by the pat 1 marked pen main bat. when the master switch mechanism is at rest, whereby the armatures 29 and 30 are attracted with the result that the battery 3 has its connections with the sides of the telephone line reversed, these connections being then traceable from the side 1 of the telephone line, the contacts 31 that are closed y the armature 30 to the negative side of the battery 3 illustrated at the right of Fig. 2 (the illustration of the battery 3 being repeated five times for sake of clearness) thence to the positive )ole of the battery which is connected with the side 2 of the telephone line through the contacts 32 closed by the armature 30. The battery 3 is thus reversed instantly upon the closure of the contacts 7 and 8 at a calling station with the results which have been described. This association of the battery and telephone line is preserved during the time the receiver at the calling station is free of its hook owing to the establishment of a locking circuit through the locking winding 33. This looking circuit may be traced from the contacts 84 (closed as a consequence of the operation of the armature 35 of the magnet 36 included in the circuit of the battery 3 last traced, the subsequent energization of the magnet 87 controlling the contacts 34) through the contacts 38 closed by the urinature 80, locking winding 33, the battery 3 and the ground that connects the battery with the contacts 34-.

When the line 1, 2 is it called line, the battery 3 is not to have its connection with such called line reversed from normal atllt) . the called line is preserved for the purpose stated owing to the inaction of the armature 30 due to the fact that the winding 28 is not energized, and the winding 33 is energized, this latter winding having operatin influence upon the armature 29 alone. W en the armature 29 is attracted, the springs 24 and 25 engage their alternate contacts whereby waiting contacts 40 and 41 of a called line are connected with the line sides 1 and 2.

The equipment illustrated in Fig. 2 will not be further described since all of the parts and the circuit relations of such parts as illustrated are now well known to those skilled in the art of automatic telephony, British Patent No. 1298 of 1910 serves to illustrate the prior art re ecting a well known type of automatic te ephone system. While have herein shown and particularly described my invention as bein embodied in an automatic telephone exc iange system in which not only trunk lines or link connectors at an exchan e are automatically constituted extensions 0 telephone lines, but in which called telephone lines are also automatically constituted extensions of called telephone lines, I do not wish to be limited to the embodiment of my invention in an automatic exchange or in an exchange in which telephone lines are automatically constituted extensions of callin telephone lines nor do I wish to be limite in all embodiments of my invention, to the employment of selectin mechanism for automatica ly extending to e hone lines. Neither do I wish to be limite to the precise circuit arrangements that I have illustrated for accomplishing the object of my invention, as changes may readily be made without d'eparting from the spirit of the invention.

Having thus described my invention, I claim as new and desire to secure by Letters Patent the following:-

1. A. telephone exchan e system including a line connecting an axe ange with a plurality of stations equipped with telephone receivers and transmitters; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a single direction; switching devices at the telep one stations for governin'g the association of telephones thereat with said line; polarized electro-magnets for prevent-in and permitting the operation of said switc ing devices and responsive to the current normally impressed uppn the line; additional switching devices at the telehone stations for governing the circuit reation of the polarized magnets, the line, and said source of current, and in turn governed by the receivers at said stations; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have the use of the line to the exclusion of the other stations.

2. A telephone exchange system including a line connecting an exchange with a plurality of stations equipped with telephone receivers and transmitters; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a single direction; switching devices at the telephone stations for governing the association of telephones thereat with said line; polarized electro-magnets for preventing and permitting the operation of said switching devices and responsive to the current normally impressed upon the line; additional switching devices at the telephone stations for governing the circuit relation of the polarized magnets, the line, and said source of current; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the-line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have the use of the line to the exclusion of the other stations.

3. A telephone exchange system including a line connecting an exchange with a plurality of stations equipped wit 1 telephone rcceivers and transmitters; a source of current normally connected with the line; switching devices at the telephone stations for governing the association of telephones thercat with said line; electro-magnets for preventin and permitting the operation of said switching devices and responsive to the current normally impressed upon the line; additional switching devices at the telephone stations for governing the connections of the magnets with the line and in turn governed by the receivers at said stations; and means at the exchange responsive to the current initially flowing over the line and in turn operating to limit the line to current to which the magnets are not responsive; whereby the station first taking the line for use may have the use of the line to the exclusion of the other stations.

4. A telephoneexchange system including a line connecting an .exe ange with a plurality of stations equipped with telephone receivers and transmittersa source .of current normally connected with the line;

switching devices at the telephone stations for governing the association of telephones thereat with said line; electro-magnets for preventing and permittin the operation of said switching devices an responsive to the current normally impressed upon the line; additional switching devices at the telephone stations for governing the connections of the magnets with the line and means at the exchange responsive to the current initially flowin over the line and in turn 0 erating to limit the line to current to w ich the magnets are not responsive, whereby the station first taking the line for use may have the use of the line to the exclusion of the other stations.

5. A telephone exchange s stem including a line connecting an exe ange with a plurality of stations equipped with telephone receivers and transmitters; a source of uni-directional current normally connected with the line to permit current to have initial flow thereover in a single direction; switching devices at the telephone stations for governin the association of telephones thereat wit said line; olarized electro-magnets for reventing an permittin the operation oi said switching devices an responsive to the current normally impressed upon the line; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first takin the line for use may have the use of the llllG to the exclusion of the other stations.

6. A telephone exchange system including a line connecting an exciange with a plurality of stations equipped with telephone receivers and transmitters; a source of current normally connected with the line; switching devices at the telephone stations for governing the association of telephones thereat with said line; electro-magnets for preventing and permittin the operation of said switching devices an responsive to the current normally im ressed upon the line; and means at the exe ange res onsive to the current initially flowing over t e line and in turn operating to limit the line to current to which the magnets are not responsive, whereby the station first taking the ine for use may have the use of the line to the exclusion of the other stations.

7. A telephone exchange system including a line connecting an exchange with a plurality of stations equipped with telephonereceivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching mechanism at the telephone stations for causing the operation of the selecting mechanism; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a sin le direction; switching devices at the te ephone stations for governing the association of the aforesaid switching mechanism with said line; olarized electro-magnets for reventing an permitting the operation 0 said switching devices and responsive to the current normally impressed upon the line; additional switching devices at the tele hone stations for governing the circuit re ation of the polarized magnets, the line, and said source of current, and in turn governed by the receivers at said stations; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have exclusive control over said selecting mecha nism.

8. A telephone exchange system including a line connecting an exchange with a plurality ocE stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching mechanism at the tele hone stations for causing the operation o the selecting mechanism; a source of uni-directional current normally connected with the line to permit current to have initial llow thereover in a single direction; switching devices at the telephone stations for governing the association of the aforesaid switching mechanism with said line; polarized electro-magncts for preventin and permitting the operation of said switching devices and responsive to the current normally impressed upon the line; additional switching devices at the tele hone stations for governing the circuit relation of the polarized magnets, the line, and said source of current; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to make the polarized mugnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

9. A telephone exchan e system including a line connecting an exciange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switchingmechanism at the telephone stations for causing the operation of the select-in mechanism; a source of current norma 1y connected with the line; switching devices at the telephone stations for governing the association of the aforesaid'switching mechanism with said line; electro-magnets for preventing and'permittin the operation 0 said switching devices an responsive to the current-normally impressed upon the line; additional switching evices at the telephone stations for governin the connections of the ma ets with the line and in turn governed y the receivers at said stations; and means at the exchange responsive to the current initially flowing over the line and in turn 0 rating to limit the line to current to w ich the magnets are not res onsive, whereby the station first taking tie line for use may have exclusive control over said selecting mechanism.

10. A telephone exchange s stem including a line connecting an exc ange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching mechanism at the telephone stations for causing the operation of the selecting mechanism; asource of current normally connected with the line; switching devices at the tele hone stations for governing the association of the aforesaid switching mechanism with said line; electro-magnets for fpreventing and permitting the operation 0 said switching devices and res onsive to the current normally impresse upon the line; additional switching devices at the telephone stations for governing the connections of the magnets with the line; and means at the exchange responsive to the current initially flowing over the line and in turn operating to limit the line to current to which the magnets are not responsive, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

11. A telephone exchange a stem including a line connecting an exciange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching mechanism at the telephone stations for causing the operation of the selecting mechanism; a source of uni-directional current normally connected with the line to permit current to have initial flow thereover in a sin le direction; switching devices at the telep one stations for governing the association of the aforesaid switching mechanism with said line; polarized electro-magnets for preventin and permitting the operation of said switching devices and responsive to the current normally impressed upon the line; and means atthe exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first takin the line for use may have exclusive contro over said selecting mechanism.

12. A telephone exchange stem including a line connecting an exc iange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching mechanism at the telephone stations for causing the operation of the selecting mechanism; a source of current normally connected with the line; switching devices at the telephone stations for governing the association of the aforesaid switching mechanism with said line; electro-magnets for preventing and permitting the operation of said switching devices and res onsive to the current normall impresse upon the line; and means at t e exchange responsive to the current initially flowing over the line and in turn oyicrati to limit the line to current to w ich t e ma ets are not responsive, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

13. A telephone exchange s stem including a line connecting an exc ange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching devices at the tele hone stations for causing the operation of tie selecting mechanism; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a single direction; polarized electro-magnets for normally preventing and serving to permit the operation of said switching devices and responsive to the current normally impressed upon the line; additional switching devices at the telephone stations for governing the circuit relation of the polarized magnets, the line, and said source of current, and in turn governed by the receivers at said stations; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supp led to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

14. A telephone exchange system including a line connecting an exchange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching devices at the tele hone stations for causmg the operation of t 1e selecting mechanism; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a single direction; polarized electro-magnets for normally preventin and serving to permit the operating of said switching devices and responsive to the currentnormall impressed upon the line; additional switc ing devices at the tele hone stations for governing the circuit re ation of the polarized magnets, the line, and said source of current; and means at the exchange responsive to the current initially flowing over the line and in turn operating to reverse the polarity of the current supplied to the line to malte the polarized magnet at the telephone stations unres onsive to current flowing over the line, w ereby the station first takin the line for use may have exclusive contro over said selecting mechanism.

15. A telephone exchange s stem including a line connecting an exe ange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switching devices at the tele hone stations for causlng the operation of t is selecting mechanism; a source of current normally connected with the line; electro-magnets for normally preventin and serving to permit the operation 0 said switching devices and responsive to the current normally impressed upon the line' additional switching devices at the telep 10118 stations for governing the connections of the magnets with the inc and in turn governed by the receivers at said stations; and means at the exchange responsive to the current initially tlowin over the line and in turn operating to limit the line to current to which the ma nets are not res onsive, whereby the station first taking the ine for use may have exclusive control over said selecting mechanism.

16. A telephone exchange s stem including a line connecting an exchange with a plurality of stations equipped with telephone receivers and transmitters; selecting mechanism at the exchange for extending the line thereat; switchin devices at the tele )honc stations for causing the operation of the selecting mechanism; a source of current normally connected with the line; electro-magnets for normally preventin and servin to permit the operation 0 said switclnng devices and responsive to the current normally impressed upon the line; ad-

ditional switching devices at the telephone stations for overning the connections of the magnets wit the line; and means at the exchange responsive to the current initially flowing over the line and in turn operating to limit the line to current to which the magnets are not responsive, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

17. A telephone exchange system including a line connecting an exchange with a plurality of stations equipped with telephone receivers and transmltters; selecting mechanism at the exchange for extending the line thereat; switching devices at the telephone stations for causing the operation of t e selecting mechanism; a source of unidirectional current normally connected with the line to permit current to have initial flow thereover in a single direction; polarized electro-magnets for normally preventin and serving to permit the operation of said switching devices and responsive to the current normally im ressed upon the line; and means at the exe iange responsive to the current initially flowing over the line and in turn operatin to reverse the polarity of the current supp ied to the line to make the polarized magnets at the telephone stations unresponsive to current flowing over the line, whereby the station first taking the line for use may have exclusive control over said selecting mechanism.

18. A telephone exchange s stem including a line connect-ing an exe iange with a plurality of stations equip ied with telephone receivers and transn'ntters; selecting mechanism at the exchange for extending the line thereat; switching devices at the. telephone stations for causing the operation of t 1e selecting mechanism; a source of current normally connected with the line; electro-magnets for normally preventin and servin to permit the operation 0? said switching devices and responsive to the current normally impressed upon the line; and means at the exchange responsive to the current initially flowin over the line and in turn operating to limit the line to current to which the magnets are not res )onsivc, whereby the station first taking the ine for use may have exclusive control over said selectin mechanism.

In witness whereof, I hereunto subscribe. my name this fourth day of May A. D.,191l.

MORTON L. JOHNSON.

Witnesses:

MARGARET D. Rona, G. L. Cnooo.

Copies of this patent may be obtained for five cents each, by addressing the Commissioner of Patents, Washington, D. G." 

